I think it's a case of mistranslation (French developers). In the French version of the game (mybrute.fr - compatible with the English version) that weapon is called a 'Marteau', which means hammer. I suppose the problem is that they called the piledriver skill 'Hammer', which I am guessing is because the French word for piledriver also contains the word Marteau as well. I mean, does that thing look anything like an 'Axe' to you?